In general, a non water-soluble or a hardly water-soluble organic acid such as a carboxylic acid is crystallized by using a reactive crystallization so-called “neutralization crystallization” in which the organic acid is crystallized by reacting its salt with an acid in the presence of water.
An example of the neutralization crystallization is a method in which crystals of a crystalline organic acid, such as adipic acid or nicotinic acid, are produced by adding an acid to a water solution of a water-soluble salt of the crystalline organic acid (See, for example, Reference 1: Fang Wang and 1 other, “Monitoring pH Swing Crystallization of Notice Acid by the Use of Attenuated Total Reflection Fourier Transform Infrared Spectrometry”, “Ind. Eng. Chem. Res. Vol. 39, No. 6, 2000”, p. 2101-2104).
In the foregoing neutralization crystallization, the crystals of the foregoing organic compound are precipitated by a) dropping an acid on a surface of an alkaline water solution of the foregoing organic compound in a container by using a pump or the like, or b) dropping the acid into the alkaline water solution of the foregoing organic compound in the container by using a dipping tube.
According to the foregoing Reference 1, as hydrochloric acid is being dropped into a water solution of sodium nicotinate, the solution transits from an unsaturated state (I) in which nicotinic acid is not yet saturated, to a supersaturated state (II) in which crystals are not precipitated even though nicotinic acid being dissolved exceeds a saturation point of nicotinic acid. Then, as hydrochloric acid is being further dropped into the solution, a rapid desupersaturation (III) of the solution occurs due to crystallization, thereby resulting in a saturated state (IV) in which the crystals are precipitated.
However, the inventors of the present invention found reaction of mono-sodium adipate with hydrochloric acid according to the method described in Reference 1 only gave such adipic acid crystals whose mean particle diameter of the crystals obtained was as small as 129 μm, and bulk density of the crystals was as small as 267 kg/m3.
As described, the conventional method of neutralization crystallization only produces crystals whose mean particle diameter and bulk density are small. This causes such a problem that, for example, long time is needed for a filtration for filtering out the crystals obtained by the crystallization.
In view of the foregoing problems, the present invention was made, and it is an object of the present invention to provide a crystallizing method for producing, in the method so-called neutralization crystallization, crystals whose mean particle diameter is larger than a conventional mean particle diameter of the crystals.